1. Field of the Invention
The present invention relates generally to an optical repeatered transmission systems, and more specifically to a fault location technique for optical repeaters and a transmission system employing such repeaters.
2. Description of the Related Art
In conventional optical repeatered transmission systems, a data signal is converted to an optical signal and transmitted to a first repeater station where the signal is converted to an electrical signal, amplified by an electronic amplifier and converted to an optical signal for transmission to a second repeater station, where the process is repeated for transmission to a distant terminal station. In conventional coaxial cable repeatered systems, on the other hand, a supervisory signal is multiplexed with a data signal and transmitted to repeater stations. At each repeater station the supervisory signal is constantly monitored. If a fault occurs in the incoming optical transmission channel of the first repeater station, the latter detects the absence of the supervisor, signal and sends out a fault-point indicating signal to the second repeater station, where it is relayed to the distant terminal station as an indication of the location of the fault.
In the optical transmission technique the general tendency is toward using an erbium-doped fiber excited by light energy supplied from an excitation laser in order to benefit from its ability to directly amplify optical signals. Because of the direct light amplification, however, the current electronic processing circuitry cannot be employed for optical repeatered transmission systems using erbium-doped fiber.